76 Experimental Zoology 



the hereditary nature of the chromosomes is that the spermato- 

 zoon brings into the egg only the chromosomes of the male 

 germ-cells. While it is undoubtedly true that the largest part 

 of the sperm-head is made up of chromatin material derived 

 from the nucleus, it is also true that the protoplasm of the origi- 

 nal germ-cell is not lost. It is partly used up in the formation 

 of the tail of the spermatozoon, but also forms a condensed 

 layer around the sperm-head. There is every reason to assume 

 that the latter may become incorporated as a part of the cyto- 

 plasm of the egg. There is also another serious objection to the 

 explanation of the purity of the germ-cells given above, for, even 

 if the chromosomes be the bearers of the hereditary qualities of 

 the egg and sperm, it does not follow that each unit character 

 would be contained in only one chromosome. If it be assumed 

 that each chromosome carries all the hereditary qualities, it is im- 

 possible to account for the purity of the germ-cells on this as- 

 sumption. For instance, if we assume that each of the chro- 

 mosomes contains all of the hereditary characters, the germ -cells 

 of the hybrid, A(B), will contain, before synapsis, half of their 

 chromosomes bearing the character A, and half bearing the 

 character B. If these unite at the synapsis in pairs, and then 

 come to lie, haphazard, on the spindle, some turned one way, 

 some the other, the resulting germ-cells will contain all 

 mixtures of A and B, and hence be impure. If we give up 

 the idea of "purity" and assume that the relative number of 

 A or of B chromosomes determine the character of the result- 

 ing cells, the three types of the Mendelian ratio might be ac- 

 counted for, provided the reduced number of the chromosomes 

 is an odd number. If it were an even number, it must often 

 happen that equal numbers of a character, of A or B, would be 

 contained in the same germ-cell, and hence there would be an 

 exact balance, which on the theory should give neither result. 

 This, however, is not in harmony with the facts. 



In the light of these theoretical difficulties it seems to me that 

 the chromosomal theory must be applied to Mendel's law with 

 caution, and that while at first sight it appears to offer an 



